REMOVAL OF THE BRAIN 203 



protrude (Fig. 83). As a general rule, they are most evident 

 towards the posterior part of the parietal region. At first sight 

 they appear to be protrusions from the dura mater, but this 

 is not the case. They spring from the arachnoid and the sub- 

 arachnoid tissue, and are enlargements of the normal villi of 

 the membrane (Fig. 84). 



Two Layers of the Dura Mater. Having noted these 

 preliminary details from an examination of the outer surface 

 of the dura mater, the student is in a position to understand 

 that this membrane does not belong entirely to the brain. 

 It performs a double function: (i) it acts as an internal 

 periosteum to the bones forming the cranial cavity ; and (2) 



Arachnoideal granulation Opening of cerebral vein 



?fHt^ J -7 -i '- .-: Bone 



FIG. 83. Median section through the Frontal Bone and corresponding part of 

 the Superior Sagittal Blood Sinus. The arachnoideal granulations are 

 seen protruding into the sinus. (Enlarged. ) 



it gives support to the different parts of the brain. Conse- 

 quently, it consists of two strata, which, in most localities, are 

 firmly adherent, but they can usually be easily demonstrated 

 in the dissecting-room. These strata may very appropriately 

 be termed the endocranial and the supporting layers. Along 

 certain lines these two layers separate from each other. In 

 some cases they separate so as to form blood channels, termed 

 sinus durce matris, for the conveyance of venous blood ; in 

 other cases they separate in order that the inner supporting 

 layer may form strong folds or partitions, which pass in 

 between certain parts of the brain ; and by these partitions the 

 cranial cavity is divided into compartments communicating 

 freely with one another, and each holding a definite sub- 

 division of the brain (Fig. 87). 



Dissection. These points must now be verified. Begin by tilting the 



